The present invention relates generally to apparatus which transmits and receives modulated signals and, more particularly, to a transceiver which is capable of determining its prescribed reception frequency by measuring the frequency of its carrier signal output.
Power line communication systems are used to communicate between stations that are operatively coupled to a power line. A typical application of a power line communication system is used by an electrical utility to communicate between a central station and a plurality of remote stations. A central communications unit is located at a central station and is operatively coupled to the power line. This central station is equipped to transmit commands to thousands of remote locations which are equipped with receivers that are also operatively coupled to the power line. At the remote locations, such as consumer residences, the receivers are typically incorporated in load management terminals. The load management terminals are connected to non-essential electrical equipment, such as air conditioners or water heaters, that can be shed during periods of peak electrical power usage. Each load management terminal is assigned a unique address in order to permit the central station to selectively send commands to a particular consumer residence or request data therefrom. In this type of system, messages can be sent from the central station to the remote station and, in some applications, vice versa. The message emanating from the central station would include load shedding commands or requests for meter readings. Return messages from the remote stations could be status reports or meter readings.
U.S. Pat. No. 4,130,874 which issued to Pai on Dec. 19, 1978 discloses a load management terminal having a plural selectable address formats for a power line communication system. A distribution network communication system having branch connected repeaters is disclosed in U.S. Pat. No. 4,250,489 which issued to Dudash on Feb. 10, 1981. Various types of load management terminals that can be used with power line communication systems are known to those skilled in the art. U.S. Pat. No. 4,429,366 which issued to Kennon on Jan. 31, 1984 discloses a microprocessor-based load management terminal with reset capability. Also, a load management terminal is disclosed in U.S. Pat. No. 4,402,059 which issued to Kennon et al. on Aug. 30, 1983. Load management terminals are provided with digital demodulators that are capable of demodulating incoming messages received from the power line. For example, a coherent phase shift keyed demodulator for power line communication systems is disclosed in U.S. Pat. No. 4,379,284 which issued to Boykin on Apr. 5, 1983 and a coherent phase shift keyed demodulator is disclosed in U.S. Pat. No. 4,311,964 which issued to Boykin on Jan. 19, 1982.
Although various techniques of power line communication systems are known to those skilled in the art, one particular technique is to modulate a carrier signal with a base band data message and inject the modulated signal onto the power line. This modulated signal would be transmitted along the power line and, when received by another station, the message would be subsequently demodulated and interpreted. Various modulation techniques are known. One particular modulation technique involves a phase shift keyed (PSK) modulation, by a base band data signal, of a higher frequency carrier signal. A typical carrier signal in this type of application would be approximately 12.5 kHz and the base band data signal would be approximately 76 baud. The base band data signal and the carrier signal, in a phase shift keyed system, are introduced as separate inputs into an exclusive-OR (EOR) gate. Changes in the logical state of the base band data message cause the phase of the carrier signal to be shifted.
When the phase shift keyed message is received by a remote station, it is demodulated and the message is decoded. A digital demodulator suitable for demodulating messages of this type is disclosed in U.S. Pat. No. 4,311,964 which issued to Boykin on Jan. 19, 1982. Improved digital demodulators are disclosed in U.S. Pat. Nos. 4,514,697 and 4,516,079 which issued to York. A more recent improved digital demodulator is disclosed in U.S. Pat. No. 4,563,650 which issued to York et al. Although these digital demodulators are particularly well suited for demodulating phase shift keyed messages, other demodulating apparatus could also be used.
Regardless of the particular demodulating technique used, the carrier frequency must be known in order that the demodulator can effectively sample the incoming signal during the demodulating process. Since the demodulator must be able to discern a valid incoming message from spurious electrical noise, precise sampling techniques are used and the results of these sampling techniques are subjected to an algorithm to determine whether a valid incoming message is being received and to demodulate the message. The known frequency of the incoming message is used to determine the period of time between samples. If the digital demodulator did not know, in advance, the frequency of the incoming message, proper demodulation of the message would be impossible.
Transceivers which are used in conjunction with power line communication systems are initially provided with information relating to the frequency of messages which will be incoming to the transceiver. If all transceivers were intended for use with a single frequency, the value of this single fixed frequency could be incorporated within the demodulating algorithm. However, since various carrier signal frequencies are used in different power line communication systems, some means must be provided for informing the digital demodulator of the exact prescribed frequency value of valid incoming messages.
Since typical power line communication devices, such as the load management terminals, comprise microprocessors which are used to demodulate and decode incoming messages, the prescribed transmission and reception frequency for the transceiver can be stored in the memory of the microprocessor. Typically, this value would be stored in a location of the microprocessor's read-only memory (ROM). Since a typical design of a load management terminal could utilize any one of a plurality, such as eight, of possible carrier signal frequencies, the stored value of the prescribed frequency for a particular transceiver would be one of eight preselected values. Although the use of read only memory (ROM) provides non-volatile storage of the prescribed frequency values which is not susceptible to loss during power outages, severe electrical transients, such as lightning, could possibly have an adverse effect on the non-volatile random access memory NOVRAM location containing the selected value. If, for some reason, the selected value of the prescribed frequency is destroyed, the digital demodulator would not be able to properly demodulate incoming carrier-based signals.
This problem can be addressed in various ways. One possible means for preventing the loss of the prescribed value during electrical transient conditions would be to fix this value with hardware. For example, three switches would provide sufficient resolution to store the binary representation for one of eight possible frequencies to designate the prescribed frequency of the transceiver. This type of hardware storage of the frequency value would be immune to electrical transient conditions. However, a significant disadvantage of this method is that both the transmitter and the receiver would have to be individually configured at additional cost and would require the use of port pins on the microprocessor that are needed for other functions. Another way to address the problem of possible loss of memory relating to the prescribed frequency value is to custom design the demodulator algorithms to be applicable to only one frequency. This technique also has severe disadvantages. The digital demodulator of the transceiver, which is typically "masked" to a read-only memory (ROM), would then be unique to a single frequency and would not be applicable to transceivers which require alternate frequencies. Therefore, no single ROM would be appropriate for general use in all transceivers. Additional costs would be incurred, not only in the individual designing of these multiple digital demodulators, but also in the inventory requirements and particularly assembly procedures that would also be necessitated.
It is therefore advantageous to provide a transceiver with the flexibility to adapt to one of a plurality of possible carrier frequencies without having to manufacture distinct transceivers for each frequency. The present invention provides a transceiver that is capable of determining its own prescribed receiver frequency following a power outage or other transient condition.
The present invention comprises a means for generating a high frequency signal that has a higher frequency than any potential carrier frequency. In a preferred embodiment of the present invention, this high frequency signal has a frequency of 1 mHz. The present invention also comprises a means for generating a signal when a negative edge of a carrier signal occurs. It should be understood that the present invention could operate alternatively by responding to a positive edge of the carrier signal, but the preferred embodiment described herein is configured to be responsive to negative edges.
The present invention also incorporates a counter which accumulates a value which is representative of the number of pulses, or changes of logical state, of the high frequency signal. By storing the value of this counter upon successive occurrences of negative edges of the carrier signal, the number of pulses that occur during one period of a carrier signal can be determined. If the period of a signal is known, the frequency of that signal can easily be calculated. More simply, since the period can be determined by multiplying the number of pulses stored in the above-described counter by the known period of the higher frequency signal, the result of this calculation can be compared to a look-up table to directly determine the frequency of the measured carrier signal. After this interval is determined, it can therefore be compared to a look-up table which contains all of the possible frequencies for the transceiver. After matching this interval with one of the frequencies of the look-up table, the prescribed frequency of the transceiver can be stored in the NOVRAM.
Since the frequency of transmission by the transceiver would be identical to the frequency of incoming signals or have a known relationship (such as .+-.2 kHz) with the frequency of incoming signals, the ability to determine its own output frequency, which is actually measured, would provide the transceiver with the ability to determine expected incoming frequency. The functions of counting the high frequency pulses, responding to the negative edges of the carrier frequency and determining the interval between successive negative edges is accomplished by a microprocessor in a preferred embodiment of the present invention.